I often look at gear valued in terms of stat weights, whether they be from SimCraft, Theck's simulations, AskMrRobot, Wowhead or wherever. It's not that I let my gear choice be dictated by them, but it can be useful to stop you overlooking a hidden gem or get a rough idea of which upgrades are biggest (e.g. when spending scarce VPs). However, most weightings - even for tanks now - put a high value on hit and expertise, because they are best for dps or active mitigation etc. However, with decent gear, you can usually reach hit and expertise caps regardless of a single change in gear, thanks to reforges and gems. So a potential new hit/expertise piece would be over-valued if one just looked at its stat values, as acquiring it would not increase your hit or expertise, but rather let you reforge into the next best stat (let's call it haste).

One fix would be to assign to hit and expertise the value of next best stat, rather than their own intrinsic value. That still may overvalue them - if sometimes you can only reforge into a weaker stat or you have so much hit/exp you are still over caps after all adjustments. But those cases may be exceptional and anyway, the adjustment is an improvement over the raw score. Am I way off base with this?

A related point is that with reforging, even your weaker stats (let's call them crit) are more valuable than their intrinsic value - as you can reforge a proportion (0.4?) to something better. Maybe they should be valued at the 0.6Xinstrinsic value + 0.4xvalue of stat can reforge into. Ideally, you'd adjust this item by item, as the stat you can reforge it would vary by item, but a quick dirty adjustment might just be to give replace 0.4 of the value with that of the best stat (e.g. haste) or an average of the value of other stats (e.g. haste + mastery).

What I am thinking of here is not a detailed comparison of two particular pieces of gear, when you would eyeball all the individual stats and not collapse them into a single stat value for a piece of gear. But rather adjustments you could make in user-input stat values for use in Pawn, Wowhead, AMR or other tools that are used to filter long gear lists. So I am not looking for a perfect or precise solution, just an approximation.

Not sure if you got your answer, but if you're using AMR, they have a "relative" option (as opposed to "absolute") that will calculate upgrades based on your current gear. Iow, if you're sitting at the hit/exp caps, it will have hit/exp pieces at a lower value (unless reforging/regemming allows for a better set up with a newly acquired hit/exp piece.)

Ah, I suspected the point was too obvious for something like AMR to overlook. I've found the relative toggle now - just what was looking for, thanks. It does shake things around a bit (and incorporate the value of tier set bonuses better).